1. Field of the Invention
The invention relates to a filter circuit which is suitable when it is applied to, for example, an integrated circuit or the like.
2. Related Background Art
There are the following reference literatures 1 to 3 as literatures describing conventional analog filter circuits.                Reference literature 1: JP-A-6-120772        Reference literature 2: JP-A-6-164314        Reference literature 3: JP-A-11-004139        
Among them, it is an object of the reference literature 1 to provide an active filter in which an amplitude range of an input signal is widened while suppressing an increase in current consumption.
It is an object of an active filter circuit of the reference literature 2 to improve an S/N ratio of the filter circuit and suppress input conversion noises.
It is an object of an active filter circuit of the reference literature 3 that, in a filter circuit of the LC ladder type constructed by a voltage controlled current source and a plurality of capacitors and with resistances terminating both ends of the ladder, the occurrence of an unnecessary time constant due to a parasitic capacitance which is formed at a node of a circuit for performing the terminating resistance operation and a circuit for performing the serial reactance operation is prevented, thereby obtaining good filter characteristics.
The following methods are considered on the basis of the contents of the reference literatures 1 to 3.
In case of forming an integrated circuit (IC) of a filter circuit of the LC ladder type with resistances terminating both ends as shown in FIG. 5, since it is difficult to construct an inductor into the IC, generally, the formation of the IC is realized by replacing the inductor with a voltage controlled current source and a capacitor. By also replacing a terminal resistor with the same device as the voltage controlled current source, a filter circuit using no resistor can be constructed.
The reason why such a replacement is performed is as follows. That is, the resistors are generally formed with variations in temperature characteristics and device values upon manufacturing of the ICs and, after the ICs were once manufactured, it is impossible to adjust the variations in the temperature characteristics and device values. On the other hand, according to the voltage controlled current source with an adjusting terminal, even after the IC was manufactured, a mutual conductance can be adjusted by using the adjusting terminal. There is, consequently, a large advantage such that even if there are variations in the temperature characteristics and device values, the characteristics of the filter circuit can be maintained.
Generally, with respect to the voltage controlled current source, the following equation is satisfied from a relation between an input voltage Vin and an output current Iout as shown in FIG. 8.Iout=Vin×gm  (1)                gm: mutual conductance of the voltage controlled current source        
For example, a case where a low pass filter circuit of the dual resistance terminating LC ladder type of the ternary π type shown in FIG. 5 is constructed by a voltage controlled current source and a capacitor will now be considered. However, it is assumed that two terminal resistors R11 and R12 have the same resistance value.
FIG. 9 is an explanatory diagram of a circuit obtained by replacing a parallel resistor circuit with a voltage controlled current source. In FIG. 9, the following equation is satisfied from the relation between an input voltage Vr and an input current Ir.Ir=Vr/R−Ir=Vr×(−gm)∴gm=1/R  (2)
FIG. 10 is an explanatory diagram of a circuit obtained by replacing a parallel inductor circuit with a voltage controlled current source and a capacitor. In FIG. 10, the following equations are satisfied from the relation between an input voltage V1 and an input current I1.Z1=LsZc=1/CsI1=V1/Z1=V1/LsIa=V1×gmIb=Ia×Zc×(−gm)=−Ia×gm/CsI1=−Ib=Ia×gm/Cs=V1×gm^2/Cs∴C=L×gm^2  (3)where,                Ls, Cs: transfer function expressions of an inductance L and a capacitance C        s: frequency obtained by more generalizing a frequency f and s=j×2πf        Zc: impedance of the capacitor C        ^2: square of a just-previous character        
FIG. 11 is an explanatory diagram of a circuit obtained by replacing a serial inductor circuit with a voltage controlled current source and a capacitor. As for the serial inductor circuit, it is sufficient that the parallel inductor circuit of two terminals shown in FIG. 10 is converted into a circuit of four terminals.
In FIG. 5, to convert the terminal resistor R11 on the input side into a parallel resistor, an input signal is converted from a voltage into a current, so that a circuit shown in FIG. 6 is obtained. Therefore, if the low pass filter circuit shown in FIG. 5 is constructed by a voltage controlled current source and a capacitor by using the parallel resistor circuit and the serial inductor circuit mentioned above, a circuit as shown in FIG. 7 is obtained. An input terminating portion 71 corresponds to R11 in FIG. 5, an inductor portion 72 corresponds to L11, and an output terminating portion 73 corresponds to R12.
Generally, since any devices (active device and passive device) generate noises, in the circuit obtained as a result of the above replacement, the voltage controlled current source itself can become a main noise source. Although various factors are considered as noises, for simplicity of explanation, it is assumed that the following relational equation is satisfied in output noises Ni of the voltage controlled current source whose mutual conductance is equal to gm.Ni=4×k×T×dF/gm  (4)                where,        k: Boltzmann's constant        T: absolute temperature        dF: frequency band width        
Since all mutual conductances of voltage controlled current sources in a filter circuit 70 in FIG. 7 are equal, output noises N7 in the filter circuit 70 are expressed by the following equation.N7=√(7×Ni^2)=√7×Ni=2.65×Ni  (5)
It is now assumed that all of the voltage controlled current sources in the filter circuit 70 uniformly exert an influence on the output noises N7. Therefore, the value of N7 does not depend on a connection of the filter circuit 70 but depends only on the number of voltage controlled current sources existing in the filter circuit 70.
Although it is effective to allow an input and an output of the filter circuit to have a gain in order to improve input conversion noises in the filter circuit, in the filter circuit with a construction as shown in FIG. 7, an input signal does not have a gain. Even if an amplifying circuit is connected to the post stage of such a filter circuit in order to allow the input signal to have a gain, not only a signal of a frequency component in a band where the filter circuit passes but also noises which are generated by the filter circuit are amplified by an amount corresponding to the same gain, so that there is not an effect of improving the input conversion noises.